In a typical cellular radio system, end-user radio or wireless terminals, also known as mobile stations and/or user equipment units (UEs), communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or an “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UEs) within range of the base stations.
In some radio access networks, several base stations may be connected, e.g., by landlines or microwave links, to a radio network controller (RNC) or a base station controller (BSC). The radio network controller supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM). UTRAN is a radio access network that uses wideband code-division multiple-access (W-CDMA) for communications between the UEs and the base stations, referred to in UTRAN standards as NodeBs.
In a forum known as the 3rd Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks generally and UTRAN specifically, and investigate techniques to enhance wireless data rates and radio capacity. 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies. Several releases for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) specification have issued, and the standards continue to evolve. The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE).
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology where the radio base station nodes are connected to a core network, via Access Gateways (AGWs), rather than to radio network controller (RNC) nodes. In general, in LTE systems the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes, referred to in the specifications for LTE as eNodeBs, and AGWs. As a result, the radio access network (RAN) of an LTE system has what is sometimes termed a “flat” architecture, including radio base station nodes that do not report to radio network controller (RNC) nodes.
Transmission and reception from a node, e.g., a radio terminal like a UE in a cellular system such as LTE, can be multiplexed in the frequency domain or in the time domain, or combinations thereof. In Frequency-Division Duplex (FDD) systems, as illustrated to the left side in FIG. 1, downlink and uplink transmission take place in different, sufficiently separated, frequency bands. In Time Division Duplex (TDD), as illustrated to the right in FIG. 1, downlink and uplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired frequency spectrum, whereas FDD requires paired frequency spectrum.
Typically, a transmitted signal in a communication system is organized in some form of frame structure. For example, LTE uses ten equally-sized subframes 0-9 of length 1 millisecond per radio frame, as illustrated in FIG. 2.
In the case of FDD operation, illustrated in the upper part of FIG. 2, there are two carrier frequencies, one for uplink transmission (fUL) and one for downlink transmission (fDL). At least with respect to the radio terminal in a cellular communication system, FDD can be either full duplex or half duplex. In the full duplex case, a terminal can transmit and receive simultaneously, while in half-duplex operation (see FIG. 1) the terminal cannot transmit and receive simultaneously (although the base station is capable of simultaneous reception/transmission, i.e., receiving from one terminal while simultaneously transmitting to another terminal). In LTE, a half-duplex radio terminal monitors/receives in the downlink except when explicitly instructed to transmit in the uplink in a particular subframe.
In the case of TDD operation (illustrated in the lower part of FIG. 2), there is only a single carrier frequency, FUL/DL, and uplink and downlink transmissions are separated in time also on a cell basis. Because the same carrier frequency is used for uplink and downlink transmission, both the base station and the mobile terminals need to switch from transmission to reception and vice versa. An important aspect of a TDD system is to provide a sufficiently large guard time where neither downlink transmissions nor uplink transmissions occur, to avoid interference between uplink and downlink transmissions. For LTE, special subframes (located in subframe 1 and, in some cases, subframe 6) provide this guard time. A TDD special subframe is split into three parts: a downlink part (DwPTS), a guard period (GP), and an uplink part (UpPTS). The remaining subframes are either allocated to uplink or downlink transmission.
Time-division duplex (TDD) operation allows for different asymmetries in terms of the amount of resources allocated for uplink and downlink transmission, respectively, by means of different downlink/uplink configurations. In LTE, there are seven different configurations, as shown in FIG. 3. Each configuration has a differing proportion of downlink and uplink subframe in each 10-millisecond radio frame. For instance, Configuration 0, illustrated at the top of the figure, has two downlink subframes and three uplink subframes in each 5-millisecond half-frame, as indicated by the notation “DL:UL 2:3”. Configurations 0, 1, and 2 have the same arrangement in each of the 5-millisecond half-frames in the radio frame, while the remaining configurations do not. Configuration 5, for example, has only a single uplink subframe and nine downlink subframes, as indicated by the notation “DL:UL 9:1.” The configurations provide a range of uplink/downlink ratios so that the system can choose the configuration that best matches the anticipated traffic load.
To avoid significant interference between downlink and uplink transmissions between different cells, neighbor cells should have the same downlink/uplink configuration. Otherwise, uplink transmission to base station 2, BS2, in one cell may interfere with downlink transmission to base station 1, BS1, in the neighboring cell (and vice versa), as illustrated in FIG. 4. In FIG. 4, the uplink transmission of the UE in the right cell, identified in the figure as mobile station 1, MS1, is interfering with the downlink reception by the UE in the left cell, MS2. To avoid this interference, downlink/uplink asymmetry typically does not vary between cells. The downlink/uplink asymmetry configuration is signaled as part of the system information and remains fixed for a long period of time.
In LTE, the downlink is based on Orthogonal Frequency-Division Multiplexing (OFDM) while the uplink is based on Discrete-Fourier-Transform-spread (DFT-spread) OFDM, also known as Single-Carrier Frequency-Division Multiple Access (SC-FDMA). Details may be found in the 3GPP document “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation,” 3GPP TS 36.211, V11.3.0, available at www.3gpp.org. The transmission-time interval (TTI) in both cases equals a subframe of 1 millisecond, which is made up of 14 OFDM symbol intervals in downlink and 14 SC-FDMA symbol intervals in uplink, given a cyclic prefix of normal length. Portions of the OFDM and SC-FDMA symbols transmitted in these symbol intervals are used to carry user data in physical channels referred to as the Physical Downlink Shared Channel (PDSCH) and Physical Uplink Shared Channel (PUSCH). In future wireless communication systems, the length of a subframe might be significantly reduced in order to reduce user data delays. Furthermore, in future wireless systems both downlink and uplink might be based on OFDM.
Important priorities for the evolution of current wireless systems and the development of future wireless communication systems are higher bitrates and shorter delays, especially as applied to small cell scenarios. Higher bitrates can be achieved by using higher carrier frequencies, for example, where wideband spectrum resources are available. Also, TDD (Time Division Duplex) has attained an increased interest. With a dynamic TDD system, i.e., a system where the TDD configuration is not necessarily static from one frame to the next, the downlink or uplink bitrate can be instantaneously increased by adaptively changing the relation between number of intervals used for downlink (from eNodeB to UE) and uplink (UE to eNodeB). Within small cells, the propagation delays will be small, such that small guard periods can be used when switching from downlink to uplink. Accordingly, improved techniques for switching between downlink and uplink in a dynamic TDD system, while maintaining minimal interference between downlink and uplink transmissions and keeping control signaling to a minimum, are required.